About 85% of human colorectal cancers (CRCs) are associated with inactivation mutations of the tumor suppressor adenomatous polyposis coli (APC) gene. Loss-of-function mutations are generally not drug-able as it is difficult to restore a function that is lost, but understanding the molecular consequences of APC mutation and how they mechanistically link to CRC development could suggest new intervention approaches for this disease. Mice study indicates that one such consequence is the induction of HDAC2 (histone deacetylase 2) expression. HDAC2 is also elevated in human colon cancers, suggesting that APC mutation-mediated HDAC2 induction is a common feature seen in both APCmin/+ mice and human CRC. Genetic knockout of the endogenous HDAC2 diminishes adenoma formation in APCmin/+ mice, indicating the critical role of HDAC2 in intestinal tumorigenesis and strongly suggesting that targeting HDAC2 could be a promising approach for CRC treatment. Targeted cancer therapy relies on a thorough characterization and validation of the cancer target, but how HDAC2 acts to promote intestinal tumorigenesis remains unclear. The best known feature of HDACs is their deacetylase activity. Until now, the role of HDAC2 in cancer is solely attributed to its deacetylase activity. However, we have recently revealed that HDAC2 possesses a deacetylase-independent sumoylation-promoting activity. Based on our published evidence and preliminary results that HDAC2 promotes sumoylation of substrates with pivotal roles in human cancer, we hypothesize that HDAC2 sumoylation-promoting activity contributes to intestinal tumorigenesis. In this application, we will first conduct detailed characterization of the motifs required for HDAC2 sumoylation-promoting activity; we will then validate the functional role of HDAC2 sumoylation- promoting activity in both cell culture and in mice. Completion of the study will reveal information essential for the design and development of inhibitors that target HDAC2 sumoylation activity. The results of the proposed experiments could add new insights into intestinal tumorigenesis, with implication for alternative approach to treating APC-mutated CRC.